Mrs Goh left home at 8 a.m. and drove towards the city. 2⁵₁₂ hour later, Mr Goh left the house and drove at an average speed of 83 km/h towards the city. When Mr Goh reached his destination at 3 p.m., Mrs Goh was 12 km away from the city. Find Mrs Goh’s average speed in km/h. Express your answer in mixed number.
1 h = 60 min 
⁵₁₂ h = ⁵₁₂ x 60 = 25 min
2⁵₁₂ = 2 h 25 min

2 h 25 min after 8 a.m. = 10.25 a.m.

Time that Mr Goh left the house = 10.25 a.m.

Time that Mr Goh took from 10.25 a.m. to 3 p.m. to reach his destination = 4 h 35 min

1 min = ¹₆₀ h
35 min = ³⁵₆₀ h = ⁷₁₂ h
4 h 35 min = 4⁷₁₂ h

Distance that Mr Goh drove
= 4⁷₁₂ x 83
= 380⁵₁₂ km

Distance that Mrs Goh drove when Mr Goh reached
= 380⁵₁₂ - 12 
= 368⁵₁₂ km

Time that Mrs Goh took to travel from 8 a.m. to 3 p.m. = 7 h

Mrs Goh's average speed
= 368⁵₁₂ ÷ 7
= 52⁵³₈₄ km/h

Answer(s): 52⁵³₈₄ km/h